Quillworts
Order Isoëtales
The quillworts are a group of grass-like lycophytes that have a deep ancestry. Modern forms are small with linear leaves living in nutrient-poor lakes and ponds. In order to identify species, spores (and a microscope) are needed. They have an underground corm, which produces tiny amounts of wood, and roots that can absorb carbon from the soil (unlike any other plant in the world). They are also able to switch between C3 photosynthesis and CAM photosynthesis, and may represent the first plants to create CAM photosynthesis.
Quillworts are cousins to the enormous scale trees of the Carboniferous coal age, retaining some of the features of this group. Taxa resembling Isoëtes date back to the Triassic Period.
Above: Isoëtes growing on the edge of an oligotrophic lake
Above: Close-up of Isoëtes growing in shallow water
Sporophyte (=spore-producing phase)
Vegetative features
Stem
Brown, lobed, corm-like stem called a rhizomorph
Tiny amounts of wood (secondary xylem) and bark (phellem) produced
Bipolar growth: stems acts as roots and shoots
Leaves
Tufted, grass-like microphylls/lycophylls
Each leaf is also a sporangium-bearing leaf (sporophyll)
Sporangium with a ligule
Isoëtes is capable of C3 photosynthesis and CAM photosynthesis (Wickell 2021).
When terrestrial, these plants absorb and fix carbon dioxide as C3 plants
When aquatic, these plants use CAM photosynthesis to concentrate CO2, since they are living in water with low amounts of carbon dioxide or bicarbonate
Roots
Roots branch dichotomously after they emerge from the rhizomorph
Roots have an air canal running through the root, similar to other wetland plants
Isoëtes can absorb carbon through roots (**unique among plants**)
Reproductive features
Spore cases (sporangia)
Sporangia are adaxial on sporophylls, like other lycopods
Each leaf of the plant is a sporophyll, with sporangia located at the base
Isoëtes is heterosporous with some leaves being megasporophylls (with a megasporangium) and others are microsporophylls (with a microsporangium)
Gametophyte (=gamete-bearing phase)
Small, but multicellular gametophytes
Mega- and micro-gametophytes are endosporic (not released from spore)
They are also heterotrophic and not photosynthetic
Supply of nutrients is limited to what is contained in the original spore.
Female gametophyte is larger and slightly emerges to expose an archegonium and rhizoids
Male gametophyte is completely retained within the microspore, and releases sperm, which swim to the female gametophyte for fertilization
Above: Isoëtes growing in a wetland
Above: Longitudinal section of Isoëtes showing the corm (A), the roots (B), young micro-sporangium (C), and mature mega-sporangium (D)
Geologic range
Rhizomorphic lycophytes, which include quillworts and lepidodendrid scale trees date back to the Late Devonian
Plants resembling the quillworts, Pleuromeiales, date back to the Triassic Period
Diversity
The Isoëtales is represented by one extant family (Isoëtaceae) and three extinct families: Chaloneriaceae †, Pleuromeiaceae †, and Nathorstianaceae †
Single living genus: Isoëtes with over 200 spp. found throughout the world
Extinct taxa
Chaloneria cormosa †
Late Pennsylvanian of Appalachian Basin
Unbranched, upright plants (~2m tall) with rounded base, bisporangiate fertile regions, and secondary growth
Leaf bases robust, parichnos present, leaf cushions absent
Exarch protostele, medullated above basal region, with radial plates of xylem parenchyma separating groups of tracheids
Leaves with blunt keel and two bands of sunken stomata on abaxial surface of lateral laminae
Thick-walled, trilete megaspores with auriculae, conforming to sporae dispersae genus Valvisisporites
Microspores trilete and bladdered often preserved in tetrads; conforming to sporae dispersae genus Endosporites
Above: Reconstruction of Chaloneria cormosa (Fig. 1, Pigg & Rothwell 1983)
Changxingia longifolia †
Upper Devonian of northern Zhejiang Province, China.
It possesses dichotomous axes
Leaves are linear and smooth sterile which may have been persistent
Leaf cushions are rhomboidal with ligule pits and oval–oblanceolate leaf scars
Leaf cushions or bases bearing a ridge are helically arranged in parastichies on wide axes
Smooth megasporophylls with vertically expanded base borne in helices include a pedicel, a heel, and an upturned linear lamina
The pedicel consists of a distinct keel and horizontal alations and the lamina has the distal part reflexed abaxially
A single ellipsoidal and sessile megasporangium occurs on the adaxial side of the pedicel and produces Lagenicula-type megaspores with delicate spines
Changxingia may be assigned to the Dichostrobiles of the Isoёtales sensu lato and is compared with related taxa of the Late Palaeozoic
Fertile units (megasporophyll–sporangium complexes) are interpreted to have functioned as spore dispersal units
Heterosporous lycopsids with monosporangiate strobili are scarce in the Devonian, and Changxingia thus contributes to the understanding of their early evolutionary history and megaspore dispersal mechanisms
Cyclomeia †
Early Triassic of Australia and Tasmania
Produced terminal bisporangiate, penduculate cones of the Skilliostrobus-type
It possessed helically-arranged, wedge-shaped sporophylls with an adaxial groove containing obovate sporangia (Taylor & Taylor 1997)
The cones was 8 cm wide, and about 4 cm long
C. longicaulis
Originally named Pleuromeia longicaulis (White 1981)
C. undulata (White 1981)
Above: Reconstructions of Cylomeia undulata and Cylomeia longicaulis (Fig 6, White 1981)
Cyclostigma kiltorkense †
Chaloner 1968; Schweitzer 1969
Late Devonian (Famennian) of Kilkenny, Ireland, and Bear Island, Norway
Up to 300 mm in length
Sporophylls are linear, up to 15 cm long
Megasporangiate strobilus
Sporangia are ellipsoidal
Megaspores are 760–1520 µm in size
Cylostrobus †
Retallack 1995
Early Triassic of Australia
Similar to Pleuromeia, but with very compact and round cones
The genus Cylostrobus was erected for the compact cone only, in the paleobotanical system of form genera, but these small plants are well enough understood that the name Cylostrobus is used for the whole plant
C. indicus
Middle Triassic
C. ornatus
Late Triassic of Argentina
Petrified cone specimens originally named Austrostrobus ornatum (Morbelli & Petriella 1973)
C. sydneyensis
Middle Triassic
Above: Sporophylls of Cyclostrobus species (Fig 1, Retallack 1997)
Cymastrobus irvingii †
Large bisporangiate cone that may exceed 8 cm long and 5 cm wide
Cone axis narrow, about 10% the width of the cone, containing a ring of primary xylem showing a corrugated outline with an almost continuous band of exarch protoxylem; sporophyll traces departing from the bays of the primary xylem cylinder.
Sporophyll-sporangium units arranged helically, about 8-10 per gyre
Sporophylls comprised of a long, narrow pedicel widening distally but without alations, and a delicate distal lamina oriented perpendicularly to the pedicel; pedicels showing an abaxial keel and a distal heel, the latter forming hexagonal shields protecting the sporangia externally
Megasporangia and microsporangia in distinct parts of the cones; megasporangia proximal, enclosing a large number of megaspores
Casts of megaspore central body up to 500 µm in diameter, showing numerous small circular pores arranged in several rows around the trilete mark, smooth elsewhere
Casts of microspore central body less than 100 µm in diameter, showing one small pore between the rays of the trilete mark.
Ferganodendron †
Dobruskina 1974
Triassic
Resembles Pleuromeisa and Sigillaria
20-30 cm in diameter and covered with numerous, elliptical-rhomdohedral leaf bases that are helically-arranged
Leaves are small and only found don distal portions
Lepacyclotes †
L. cicrcularis
L. convexus
L. ermayinensis
L. zeilleri
Above: Sporophylls of Leptacyclotes species (Fig 1, Retallack 1997)
Leptophloeum rhombicum †
Late Devonian (Frasnian) Huangchiateng Formation of Hubei, China
300–400 mm in width; 10–25 m in length
This plant shows the lepidodendroid phyllotaxy
The leaf cushion inter-areas are absent.
Leaf cushions are rhombic, about 8–10 mm high and 10–12 mm wide, and the height-to-width ratios appear to be higher in the lower part of the trunk.
In a few instances, a tiny dent can be observed in the upper corner of leaf cushions, possibly representing a ligule pit.
There is an oval leaf scar about 3.0–3.5 mm long and 1.5–2.0 mm wide, approximately in the center of the leaf cushion.
Sporophylls are peltate in outline aggregated into a strobilus
Sporangia are elongate to ellipsoidal
Lycaugea edieae †
Late Devonian (Famennian) of Australia
Axis with helically arranged deciduous leaves
Leaf bases showing a parichnos below a small leaf vascular trace.
Leaf trace and parichnos located in the upper part of the leaf base.
Parichnos in leaf bases large, filled with contiguous parenchyma cells showing thicker walls towards the outer region of the leaf bases. Primary vascular tissues consisting of a medullated stele.
Protoxylem continuous, forming a thin ring with a smooth outer border around the metaxylem.
Primary cortex three-zoned, with a wide middle cortex and a homogeneous outer cortex bounded externally by a hypodermis consisting of narrow cells with heavily thickened walls
Leaf traces in the outer cortex associated with the radially elongated abaxial cavity of the parichnos.
Above: Schematic view of stem cross-section of Lycaugea edieae (Fig 1, Meyer-Berthaud et al. 2021)
Lycomeia rossica †
Neuburg 1960; Dobruskina 1985
late Early Triassic or early Middle Triassic
Originally called Pleuromeia rossica
Lycostrobus †
Late Triassic
Form genus for isolated cones thought to be associated with Pleuromeiales
L. chinleanus
L. scottii
Minostrobus chaohuensis †
Wang 2001; Wang et al. 2012
Late Devonian of Anhui, South China
Aerial axes multi-dichotomous
Leaf persistent, simple, linear, with single mid-vein.
Fertile portion with two separate monosporangiate strobili.
Megasporangiate and microsporangiate strobili monoecious, with six sporophylls per gyre.
Sporophyll with single vein, consisting of a lamina with a pointed apex and smooth margins, associated with a single adaxial sporangium.
Sporangia spherical to spherical-elliptical in outline. Each megasporangium with four megaspores, megaspore body with biform ornaments.
Solid exarch primary xylem.
Protoxylem confined to twelve ridges at the peripheral of primary xylem strands.
Metaxylem tracheids bearing Williamson’s striations
Nathorstia †
Pleuromeria †
Triassic of Germany, France, Spain, Russia, China, and Australia
It was an opportunistic pioneer plant that grew on coastal mineral soils (halophytic) with little competition as monotypic stands (Retallack 1997)
Some specimens from China indicate that those species lived in more xeric, inland sites, near desert oases (Wang & Wang 1982)
Pleuromeia was a herbaceous plant that probably lacks secondary tissues, although there may be some evidence for secondary cortical tissues
and has an unbranched stem of 30 cm long and 2–3 cm wide in the earliest species, to around 2 meters long in later species
The stem supported microphylls that are discarded in the lower part of the stem
It had a 2-4 lobed bulbous base to which numerous adventive roots are attached
Pleuromeia produced a single large cone at the tip of the stem or in some species many smaller cones
The top of the cone carries microsporophylls, the lower part megasporophylls, and both types may be interspersed in the middle.
Sporophylls are shed from the bottom up
Both types are obovate, with a round to ovoid sporangium and a ligule nearer to the tip on the upper/inner side
The trilete microspores are hollow, round and 30–40 μm in diameter
Megaspores have a layered outer skin with a small trilete mark, are also hollow, round to ovoid and up to 300–400 μm in diameter
The anatomy of the spores in Pleuromeia is comparable to that of Isoëtes and substantiates the assumed close relationship between the Pleuromeiaceae and the Isoëtaceae.
P. dubia (Seward; Retallack 1995)
Late Early Triassic
P. epicharis (Wang & Wang 1990)
Triassic of Shiqianfeng Group of north China
P. hataii (Kon'no 1973)
Late Early Triassic
P. jiaochengensis (Wang & Wang 1982)
Early Triassic of Shanxi Province, China
Smallest species known, at only 50cm tall with 3mm long microphylls
P. rossica (Neuburg, 1960)
Transferred to the genus Lycomeia (Dobruskina 1985)
P. sternbergii (Corda 1839; Grauvogel-Stamm 1990)
Early Triassic of Germany
Covered in leaf bases to near the base of the plant
Exhibited 2 types of leaves in a lax position (subhorizontal)
Above: Reconstructions of three coexisting cormose lycopsids from the Early- to Middle- Triassic of Australia (Fig 9, Retallack 1997)
Sublepidodendron
Nathorst 1920; Hirmer 1927
Early Carboniferous of Norway, China
impressions of very thin bark layers or entire tubes of bark that remained when the plant organs were macerated prior to fossilization
Arborescent,heterosporous lycopsid known from trunk, branches, and cones
Leaf bases are spirally arranged, fusiform in outline, with a vascular bundle scar and keel
The trunk has an intrastelar parenchyma concentration (pith), exarch primary xylem, and secondary xylem
The branch anatomy varies from exarch primary xylem with a small, centrally located pith, to a solid exarch primary xylem strand
S. grabaui (Wang and Xu 2005)
Famennian of Jiangsu, South China
S. mirabile (Nathorst; Hirmer 1927)
S. songziense (Chen 1977; Wang et al. 2003)
Late Devonian of China
Monocaulous trunk growing from a stigmarian rhizomorph, the trunk bearing biseriate, sub-opposite to possibly alternate lateral branches that expand by means of isotomous to slightly anisotomous dichotomies, forming an excurrent canopy
Tomiostrobus †
Early Triassic of Australia and Russia
This plant was especially widespread in the aftermath of Permian Triassic mass extinctions
Tomiostrobus is preserved as whole plants closely spaced within bedding planes, and lived as an early successional weed in lake and pond sedimentary environments, like living Isoëtes
Unlike living Isoëtes, Tomiostrobus formed closed cones with sporophylls that were distinctly shouldered and woody
This may have been an adaptation to heavy grazing by herbivorous therapsids (Retallack 1997)
T. australis
T. gorskyi
T. migayi
T. mirabilis
T. polaris
T. radiatus
T. taimyrica
